Footing for wind turbine towers

ABSTRACT

Footing used for the foundation of a wind turbine tower characterized because it provides a stable horizontal platform ( 1 ) located at the surface, upon which the tower sections and the heavy elements that are subsequently mounted, viz., gearbox, generator, hub and blades, can be set without sinking. 
     The footing is characterized by its inverted truncated cone shape, and by the savings in material estimated at 20%, as well as for its union with the tower ( 4 ) through a pedestal ( 3 ) and its corresponding embedded tensioning elements in the footing.

OBJECT OF THE INVENTION

The present invention refers to a footing used in the tower foundation.This footing provides a stable surface for placement and handling of, onthe one hand, the various sections of the tower during their assembly,and, on the other, the heavy elements included inside the wind turbine.

BACKGROUND OF THE INVENTION

Wind turbine towers stand on a ground supported by a footing. Thefootings used differ in design and sizes, though the tendency is to saveon material as much as possible while still conserving the featuresnecessary to support the tower, nacelle and other elements making up thewind tower. The material used for composing the footing is concrete,which is poured directly into the hole dug into the ground and allowedto set prior to erecting the tower. Once the tower has been completed,the nacelle is brought up followed by the heavy elements, the hub andblades.

The tower comprises various sectors that are gradually assembled. Acrane is employed for putting these sectors together, hoisting each onefrom the ground and assembling them in turn until finished.

The heavy elements inside the nacelle (generator, gearbox), the hub, thetower sectors, etc., are accumulated next to the tower while waiting fortheir turn to be mounted and, in order to facilitate their handling andhoisting, they must be placed upon a flat and solid surface. Currently,the ground surrounding the tower is leveled and adapted on a surfaceequivalent to the extension reach as the footing. Notwithstanding thissolution, it is not deemed to be an ideal solution.

Along these lines, Patent EP1526278 describes a concrete foundationfooting for a wind turbine tower that determines a main part connectedto foundation piles and comprises a cylindrical part and a frustoconicalpart. This main part is in turn covered around its circumferentialborders with ballast material such as sand, onto which a layer of movedearth is placed so as to integrate the foundation with the surroundinglandscape.

This solution does not take into account that setting tower sections,which are in line to be hoisted, onto the ground around the footingcauses this area to sink, which consequently calls for the cementationof a platform around the tower with the subsequent rise in the civilengineering price for wind turbine.

DESCRIPTION OF THE INVENTION

An object of the invention is to create a footing, preferably ofconcrete, so as to erect a tower comprising different sectors, thenacelle and its corresponding internal elements, the hub and the blades.

Another object of the invention is to furnish the footing with a broadradial surface with respect to the central point, on which the tower iserected.

And finally, another object of this invention is to furnish the towerpedestal with a footing having a horizontal extension close to groundlevel with a sufficient consistency so as to fulfill its purpose ofsupporting the different tower sections during assembly.

One of the primary advantages of the present footing is the minimizingof material used, which benefits in terms of considerable savings,setting time and consequently assembly, which results in cost benefitsfor material and time.

The foregoing is attained with a footing having the shape of a truncatedcone, set inverted underneath the ground. Thus, the larger base iscloser to the surface, while the smaller base is set deep into theground. With this new arrangement, the estimated material savings is20%. Additional costs for current footings include the slabs added tothe surface to make the upper part of the foundation flat and allow forthe setting of cranes, tower sections and heavy elements such as thegearbox, generator, hub, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures have been attached with a view to explaining howthe footing is constituted, as well as its positioning with respect tothe floor:

FIG. 1 shows an overhead and plant view of the footing and itspositioning with respect to the ground.

FIG. 2 shows a general view of a wind turbine and its foundation withthe footing, object of this invention.

FIG. 3 represents a detail of the footing and its union with theconcrete tower.

DESCRIPTION OF THE PREFERENTIAL EMBODIMENT

As shown in FIG. 1 a, the footing seen from above presents a horizontalsurface (1) having a circular or any other polyhedron shape with morethan two sides obtained from a discretization of a circumference(hexagon, octagon, etc.), which extends radially from the axis (2) at alength of L. If a section were made as illustrated in FIG. 1 b, thelower part of the footing would form a truncated cone inverted at aheight H. The part composed in this way is gapless compact reinforcedconcrete. The upper horizontal part (1) has a pedestal (3) that connectswith the tower (not shown in the figure).

FIG. 2 shows the footing joined to the tower (4) and wind turbine (5).The horizontal surface (1) permits the setting down of tower sectionsand heavy elements such as the gearbox, generator, hub, etc. Therelationship between L and H is given by the conditions of the ground.There are highly resistant terrains (largely made up of rock) requiringa horizontal surface (1) with a length 2L of approximately 20 meters.Contrariwise, weaker terrains call for a horizontal surface (1) withdimensions for 2L closer to 30 meters so that the footing is firmly set.For the foregoing, distance L is equal to the radius of thecircumference that circumscribes any possible polyhedron and can varybetween a minimum L of 7.5 meters and a maximum L of 15 meters. Inaddition, height H has minimum and maximum values of 1.5 and 5 metersrespectively.

The pedestal (3) for connection with the tower (4) is at ground level,so as to cover the entire horizontal surface of the footing (1) with alayer of compact and tamped material.

Turning to the constitution of the footing, the first step involvesexcavating based on the type of terrain with a view to having the baseof the excavation in the shape of the footing, or filling it in tocreate the desired geometry. In sloping areas, spots with loose groundmust be compacted well, since these are the areas where maximum tensionoccurs.

The manner to proceed is as follows: first use the concrete base,adhering to the shape of the footing. Next in line to be placed are thelower rebars, the bar cage, the footing cut rebars and the braces aroundthe bars; lastly placing the upper rebar. Rebar placing is followed byconcrete pouring. The last stage consists of concrete setting andcuring.

As shown in FIG. 3, the footing is covered with a layer of earth (6)which is compacted once the construction process has been completed. Thepedestal (3) remains above the surface with its bars (7), whichconstitute the connection system for the tower (4) with the foundation.These connection systems are non-adhering tensioning elements embeddedinto the footing. Once the first section of the tower (8) has beenplaced onto the pedestal, the bars (7) are then anchored. In themeanwhile, the second tower section (9) is located on the layer ofcompact ground (6) and there is no danger that it will sink, since thehorizontal surface (1) of the footing provides it with a suitable seat.

1. Footing for a wind turbine tower characterized by being furnishedwith: an upper horizontal part (1) extending radially from the axis (2)and passing through the center of the tower for a distance L, a secondlower part in the shape of an inverted truncated cone having a height Hand forming a single piece without internal gaps, completely compactedand lastly, both parts remain connected by the setting of theirconstituent material.
 2. Footing for a wind turbine tower according tothe first claim, characterized because the upper horizontal surface (1)determines a circle or polyhedron shape of more than two sides. 3.Footing for a wind turbine tower according to the first claim,characterized because the material composing the footing is concretepoured into a rebar structure supported by the prepared terrain. 4.Footing for a wind turbine tower according to the first claim,characterized because the distance L can vary between 7.5 and 15 meterswhile the height H can have values between 1.5 and 5 meters.
 5. Footingfor a wind turbine tower according to the first claim, characterizedbecause the foundation is covered with a layer of earth (6) which iscompacted once the construction process has been completed.
 6. Footingfor a wind turbine tower according to the first claim, characterizedbecause the connection with the first section of the tower (8) is madewith a pedestal (3) by some bars (7) embedded in the footing.